The roof is a major portion of the surface area in building structures, accounting for as much as 40% of the surface area. The most common roof style for high-rise and industrial buildings and row homes is the flat or low sloped roof. Although nominally flat, the roof usually has a slight slope or pitch to improve drainage.
A low sloped roof comprises at a minimum a deck and a waterproof membrane. An insulation layer can be installed between the deck and the membrane, if desired.
There are two basic categories of low sloped roof construction. The conventional technique is the built-up roof system (BUR), in which layers of felt and bitumen are layered to form a membrane. A layer of gravel or a coating is placed on top to protect the membrane from ultraviolet rays. The second and more recent construction is the single-ply roofing system (SPM), in which a single elastomeric sheet overlies the deck.
The primary purpose of a roof is to separate the exterior atmosphere from the interior of the building, and maintain the integrity of that separation during expected extremes of ambient weather conditions throughout a reasonable lifetime. This requirement leads to several design factors, which include: (1) external and internal temperatures; (2) external moisture, air moisture, rain, snow, sleet and hail; (3) wind uplift of the membrane; (4) impact resistance to weather and other effects such as dropped tools and walking; (5) the esthetics of the roof; and (6) influence of solar radiation and ultraviolet rays.
Of these design criteria, the ability of the roof to withstand the effect of the wind is one of the more critical factors in low sloped roofs. The wind can cause the uplift billowing of the membrane resulting in failure, the scattering of ballast, and even catastrophic roof failure in extreme situations. However, the present methods of preventing wind uplift share one or more shortcomings, as described below.
One of the most common methods of countering uplift forces on a single-ply roofing system is the use of stone ballast. In such a method, the waterproofing membrane is completely covered with a uniform layer of stone aggregate (usually 3/4" to 21/2" diameter in size), at layer depth to produce a down-load pressure of approximately 10 pounds per square foot. The substantial weight of this aggregate must be factored into the design of the building as added load to the roof and support structure. However, the main problem with stone ballast is that strong wind forces often cause the aggregate stones to shift position, clustering in some areas and leaving other areas uncover. This phenomena is referred to as scouring. Where the migration of the aggregate results in areas that are clear of ballast the membrane can billow upward from the aerodynamic lift of the wind, resulting in the membrane becoming damaged or disengaged. In some instances, uplift and billowing will cause ballast to be propelled from the rooftop, resulting in potential damage or injury to property or persons.
Another single-ply roofing system involves mechanically affixing the waterproofing membrane and underlying insulations to the deck with fasteners, which transfer the uplift load to the deck. In a majority of commercial and industrial buildings, the deck is formed of corrugated steel of 18 to 22 gauge thickness. Decks may also be formed from wood, concrete, gypsum and other suitable materials. The fasteners experience lateral and vertical loads induced by wind uplift forces, including the oscillating loads of membrane billowing and deck flutter. This causes the fasteners to become disengaged, ultimately backing out and leaving the membrane unsecured. A backed-out fastener has a free end capable of puncturing of the waterproofing membrane, which results in moisture having direct access to the structural deck and corroding the structural steel. Additionally, when the fasteners become disengaged, the membrane billowing can increase the forces acting on the membrane seams, therein resulting in seam failure.
Another securement system used for a single-ply roofs is to fully adhere the waterproofing membrane to the top surface of a subcomponent which has been mechanically affixed to the roof deck. In this method, the membrane becomes part of the subcomponent which is subject to pressure differentials and resultant forces between the inside and outside of the structure. The subcomponents must be mechanically affixed to the structural deck by fasteners to resist uplift forces. The adhesive bond between waterproofing membrane and subcomponent top surface is subjected to shear forces as a result of expansion and contraction of the membrane. The subcomponent structure, usually layers of insulating materials, is sensitive to moisture and condensation, permitting separation of subcomponent's top surface at the interface of the adhesive bond with the membrane. The adhesives are also sensitive to moisture and temperature. An adhesive bond failure results in the loss of membrane securement.
The built-up roof system has problems similar to the fully adhered single-ply roof, in that the built-up layers of felt are secured by bitumen (asphalt). The layers can delaminate, and chunks of asphalt/felt can be blown off the roof. The built-up roof system also shares the problem of gravel scouring with the single-ply roofing system.
One method of reducing uplift in situations where insulation panels are installed on top of the membrane is disclosed in U.S. Pat. No. 4,583,337, which teaches installing corrugated cover members along the periphery of the roof overlying insulation panels. The corrugated cover members purport to create a vacuum under the cover members by the wind flowing around the cover members. The differential pressure created by the vacuum pulls the cover members downward to retain the insulation elements.
U.S. Pat. No. 4,926,596 discloses an apertured overlay that is stretched over the membrane. The apertured overlay is secured at the periphery of the roof, and allows wind to pass through to the membrane. The overlay physically restrains the waterproof membrane from bellowing.
From the above problems and attempts to alleviate them, it is apparent that there is a desire to have a structure and/or method of reducing uplift on the membrane, and of limiting scouring of ballast.